This invention relates to semiconductor injection lasers having integrated optical injection means to enhance their response time to pulsed current modulation.
Because of their small size, high efficiency, large scale integration capability and direct modulation capability, semiconductor lasers are highly desirable as transmitters for integrated high speed optical communication systems.
Unfortunately, the rate of direct modulation is limited by serious distortion of the radiation output waveform caused by relaxation oscillations of the radiation intensity output upon modulation of the laser. Thus, for practical applications of modulation, the modulation rate is bounded by the relaxation oscillation frequency.
Several solutions have been proposed to overcome or suppress the relaxation oscillations in semiconductor lasers. These solutions have all required auxiliary optical components which are used to inject external light into the modulated laser. The injected light (1) lowers the intensity pulse amplitude at the onset of lasing, (2) increases the damping of the residual relaxation oscillations and (3) eliminates the peak in the laser frequency response characteristic. As an alternative, a bias current can also be used to improve the modulation characteristics. However, the last two mentioned changes (2) and (3) do not take place if a bias current, rather than optical radiation injection, is employed to reduce relaxation oscillation.
Illustrations are found in U.S. Pat. Nos. 4,038,610, 4,101,845 and 3,295,911, and in published articles of P. T. Ho, L. A. Glassen, E. P. Ippen and H. P. Hans in Applied Physics Letters, Volume, 33, page 241 (1978); of P. T. Ho in Electronics Letters, Volume 15, page 526 (1979); and of L. Figueroa, K. Y. Lau and A. Yarin in Applied Physics Letters, Volume 36, page 248 (1980); R. Lang and K. Kobayashi in the IEEE Journal of Quantum Electronics in Volume QE-12, page 194 (1976) and in Volume QE-11, page 600 (1975); P. Russer, Laser 1975 Optoelectronics Conference Proceedings (Munchen, Germany, 1975), page 161 and H. Hillbrand and P. Russer, Electronics Letter, Volume 11, page 372 (1975).